A. Field
The present invention relates generally to a range of polymer electrolytes characterized by high ionic conductivity at room temperature and below, improved stability, and ability to be formed in very thin film configuration, for use in lithium ion batteries, and to methods of manufacturing lithium ion batteries comprising such polymer electrolytes.
B. Prior Art
A high energy density rechargeable battery system is currently a highly sought technology objective because of the proliferation of power-consuming portable electronics that demand increasingly greater energy levels, as well as more interest in practical electric-powered vehicles with significantly improved range presently unavailable from lead acid batteries. As a result, lithium rechargeable batteries are the focus of intense investigation around the world. Table I, below, describes the available rechargeable lithium systems which are either in commercial production or under development today. The lithium solid-state polymer electrolyte battery (system 3 in the Table) would be the ideal system for such high power-consumption applications owing to its true flexibility and energy density together with a capability of very high cycle life. However, in its present stage of development, this otherwise 20 enviable system is not viable at temperatures below 60xc2x0 C.
The lithium ion liquid electrolyte battery (system 1) is presently the only commercial chemistry described in Table 1. No generic lithium ion chemistry exists since each manufacturer has its own chemistry containing different positives, different negatives, binders, electrolyte and formation process. These are major factors influencing cycle life and the charge and discharge profiles. The most common lithium sink (i.e., place where the ion inserts) negative electrodes in a lithium ion battery are carbon-type insertion compounds, while layered metal oxides of the LiMO2 type (where M=Ni or Co) or spinel lithium manganese oxides of the LiMn2O4 type are currently used as preferred lithium source positive electrodes. These electrodes are usually calendared onto metallic current collectors (which are about 25 to 50 microns thick). The overall process of these batteries may be written as: 
As indicated by the above cell reaction, charge and discharge proceed via intercalation of lithium ions into the carbon and metal oxide structure, respectively. Cell voltage at full charge is usually 4.2 volts while cell voltage on discharge is 2.6 volts.
A microporous polypropylene or polyethylene separator separates the two electrodes from shorting electrically, and liquid organic solvents containing a lithium salt as the electrolyte which is usually absorbed into the separator material and portions of the electrode provides high ionic conductivity (10xe2x88x923 to 10xe2x88x922 S/cm) and ease of migration of ions between the electrodes of the cell. These batteries are commonly used in portable computers, cellular telephones and camcorders, among other applications. The specific energy and energy density of the lithium ion battery is usually about 125 Wh/kg and 260 Wh/l, respectively. The packaged battery, usually in a hard plastic case, has a much lower energy density than the individual cell (approximately 20% lower). The cycle life (i.e., the number of times the battery can be recharged) of this battery is about 500 to 800 cycles, the self-discharge (i.e., loss of capacity on standing) per month is about 10%, and the cost is currently about $1.00 per Watt-hour of energy. These batteries can be manufactured in near fully automated, high volume production. Although lithium ion battery technology is being commercialized very heavily, numerous safety issues have arisen. For example, cells that are abused under crush test or high temperature test have been known to explode and ignite.
To overcome the disadvantages inherent in liquid electrolytes, and to obtain superior long-term storage stability, an interest has arisen in solid polymeric electrolytes in which ion mobility is achieved through coordination by sites on the polymer chain of electrolyte ions, thus promoting electrolyte dissolution and salt dissociation. An all-solid- state battery using an ionically conductive polymer membrane as the electrolyte would have several attractive features. It could be produced in virtually any shape and size, in thin film power cells or thick film energy cells, by automated fabrication techniques, as well as made reasonably rugged and leakproof, with low self-discharge, and have high open-circuit potential using a lithium metal anode. Such features represent a unique combination of properties and give rise to the possibility of using them as either secondary or primary devices across a wide range of applications.
Polyethylene oxide (PEO), a polymer examined extensively for the present application, is able to form stable complexes with a number of salts. Because of its low ionic conductivity of about 10xe2x88x929 to 10xe2x88x928 S/cm at ambient temperature, batteries using this material were found to require being operated at a temperature of 100xc2x0 C. or higher. A major problem observed with PEO-based electrolytes at temperatures below 60xc2x0 C. is their high crystallinity and associated low ion mobility. The crystalline structure of many polymers, including PEO, results in a weaker structure. In recent years, many radically different approaches have been taken to improve the conductivity of PEO and PEO-based polymers, which have also led to the proposal of other polymers for this purpose. These approaches included modification of existing polymers, synthesis of new polymers, forming composite polymers with ceramic materials, using plasticizer salts to increase ion transport and mobility of the cation, using plasticizing solvents in the polymer again to increase the ionic character of the cation, and other approaches. Several review articles describe these approaches in detail, e.g., xe2x80x9cTechnology Assessment of lithium PolymerElectrolyte Secondary Batteries,xe2x80x9d by M. Z. A. Munshi, Chapter 19 in Handbook of Solid State Batteries and Capacitors, Ed. M. Z. A. Munshi (World Scientific Pub. Singapore), 1995, and A. Hooper, M. Gauthier, and A. Belanger, in: xe2x80x9cElectrochemical Science and Technology of Polymersxe2x80x942,xe2x80x9d Ed. R. G. Linford (Elsevier Applied Science, London), 1987.
These approaches have not resulted in adequate conductivity enhancements on the polymer electrolytes desired for battery operation at room temperature. As a result, another approach has been taken in which plasticizing solvents or low molecular weight polymers are added to the polymer electrolyte to increase ionic conductivity of the PEO-based polymer electrolyte. The purpose of the latter is to increase the ionic mobility and concentrations of the charge carriers in the solid polymer electrolyte by enhancing the dissociation of the lithium salt. It is believed that the lithium ion is also solvated to the solvent molecule and participates in enhancing the ionic mobility. Many electrolyte composites incorporating low molecular weight polymers or liquid organic solvents have been prepared and have demonstrated high room temperature conductivity approaching those of the typical non-aqueous liquid electrolytes.
For example, Kelly et al, in J. Power Sources, Vol. 14, page 13 (1985) disclosed that adding 20 mole percent of liquid polyethylene glycol dimethyl ether polymer (PEGDME) to solid PEO polymer results in an increase in the ionic conductivity of the final plasticized polymer from 3xc3x9710xe2x88x927 S/cm to 10xe2x88x924 S/cm at 40xc2x0 C. However, it was found that the mechanical property of this material is very poor. In U.S. Pat. No. 4,654,279 (1987), Bauer et al disclosed the thermal cross-linking of polymers consisting of epoxies and methacrylates and plasticized with a solution of LiClO4 in a 400 MW PEG resulted in a conductivity of 4xc3x9710xe2x88x924 S/cm at 25xc2x0 C. The ""279 patent describes a polymeric electrolyte consisting of a two phase interpenetrating network (IPN) of a mechanically supporting phase of a continuous network of a cross-linked polymer and an ionically conducting phase comprising a metal salt and a liquid polymer such as liquid PEG.
Many of these low molecular weight polymers have a relatively low dielectric constant when compared to their liquid solvent counterpart, and thus limit the number of charge carriers in the plasticized polymer. In an effort to overcome this hindrance, high dielectric constant liquid organic solvent such as ethylene carbonate (EC) and propylene carbonate (PC) have been incorporated into the host polymer. The purpose was both to increase the number of charge carriers and further increase the room temperature conductivity of the polymer. The use of these organic solvents to plasticize polymers such as poly(vinyl acetal), poly(acrylonitrile), poly(vinyl acetate), and hexafluoropropene- vinylidene fluoride copolymer (Viton(trademark)) occurred as early as 1975 (see Feuillade and Perche, Journal of Applied Electrochemistry, Vol. 5, page 63 (1975)). However, the mechanical properties of these polymers were so poor that they had to be supported on porous matrices. Later, Armand produced a system with good room temperature conductivity (10xe2x88x924 S/cm) and good mechanical properties by cross-linking the Viton(trademark) and plasticizing with a solution of 1M of LiClO4 in PC (Proc. Workshop on Li Non-Aqueous Battery Electrochemistry, The Electrochemical Soc., Vol. 80-7, page 261 (1980)). Polyvinylidene fluoride (PVDF) and polyacrylonitrile (PAN) were evaluated in the early 1980s and have also been doped with a variety of liquid polar solvent, yielding room temperature conductivities as high as 10xe2x88x923 S/cm. Subsequently, PVDF has been the subject of a recent patent of Bellcore (U.S. Pat. No. 5,296,318).
The use of PC in an jonically conductive matrix containing oxygen donor atoms such as PEO complexed with a lithium salt was first presented by the applicant herein (see paper presented at the Fall Meeting of the Electrochemical Soc., Oct. 18-23, 1987). Although room temperature battery performance data was presented, the polymer electrolyte did not exhibit good mechanical property.
In the late 1980s, a series of patents were issued to MHB Inc., generally relating to the use of liquid organic solvents in various types of polymeric materials including PEO, materials based on acrylates, and low MW PEG acrylates. These patents describe predominantly radiation curing methods for the preparation of an interpenetrating polymeric network (IPN) containing various types of polyacrylates and liquid organic solvents. Although electron beam curing was stated to be the preferred method to polymerize the IPN, thermal and ultraviolet curing methods were also proposed. It was thought that containing the PC solution in the matrix of the polymeric network would therefore yield a high ionic conductivity comparable to that of PC itself. Indeed, this was demonstrated in typical polymeric networks yielding conductivities of about 2xc3x9710xe2x88x923 S/cm at room temperature. However, with solvent contents in the 60% to 80% range, oozing of the liquid was a major problem. An advantage with using electron beam curing compared to UV radiation is that the electron beam can penetrate through metallic components, and hence complete prototype cells can be made and polymer electrolyte cured in-situ. An advantage with acrylates is their ability to hold the solvents fairly well, but mechanically they are very weak. Stronger designs of acrylates have poor solvent retention capabilities. Acrylates are also good ionic conductors without solvents.
An offshoot of the lithium ion liquid electrolyte system is the lithium ion polymer electrolyte battery (system 2 in Table 1, above) that has been in development for the past four to five years. Lithium ion cells utilizing gelled electrolytes offer all the advantages of lithium ion liquid electrolyte cells and are becoming widely accepted by many companies not only because they potentially offer good form factors for a large variety of consumer electronics devices such as slim notebook computers and cellular telephones, but because they also offer improved safety over liquid electrolyte cells. The electrode chemistry is the same, but the liquid electrolyte (up to 70%) in this case is absorbed in a polymer membrane instead of the microporous polypropylene separator. The current technology based on liquid organic solvents absorbed in polyvinylidene fluoride (PVDF) polymer developed at Bellcore under U.S. Pat. No. 5,296,318 ensures good interfacial contact, which leads to relatively low internal cell resistance and, thus, good rate capability and long cycle life (up to 2500 cycles).
The current method of fabricating the polymer-solvent electrolyte involves a complex process in which PVDF is cast from a plasticizer solution of PVDF and DBP (di-butyl phthalate) to create some porosity for the liquid organic solvent. The DBP is then removed using either methanol or di-ethyl ether. The liquid organic solvent is then added to this polymer. This process is very expensive and involves hazardous chemicals.
Ironically, PVDF is non-conducting compared to many of the above-mentioned polymers, and consequently merely holds the liquid organic solvents in its structure similar to a sponge holding water. Because the technology uses an extensive amount of liquid electrolyte solvent absorbed in a polymer, it is not easy to manufacture cells at high speed. Automation of this technology may be very difficult. The gelled electrolyte cells incorporate very thick electrode/electrolyte structures (50 to 75 microns) onto metallic current collectors (25-50 microns) that not only add unnecessary weight and volume to the battery, but result in a lower cell performance. It is believed that many users incorporate an expanded gauze made of copper (anode) and aluminum (cathode) to coat the electrodes, instead of planar copper and aluminum foils. This adds more weight and volume to the already large percentage of inactive components of the cell. Present indications regarding this technology from various sources are that the energy density (gravimetric and volumetric) are lower than the existing lithium ion batteries, cycle life is not particularly impressive, and cell cost runs several dollars per Watt-hour. Like PEO, PVDF is highly crystalline, thus weakening its strength. On the other hand, acrylates are amorphous and can hold solvents well because of their ability to be cured in-situ, thus xe2x80x9ctrappingxe2x80x9d the solvents into the polymer matrix.
When this technology emerged, form factors and flexibility were among its most praised features, but currently it is used to manufacture only flat prismatic cells which exhibit little flexibility. Although scientific articles have been published asserting that such polymer electrolyte batteries can be produced in very thin film form with flexibility, these batteries tend to lose performance over time when the cell is oriented because the solvent is not completely immobilized in the polymer electrolyte. In a battery standing upright, the liquid solvents travel to the bottom of the cell, and during charge and discharge the current along the cell height will differ because of the difference in conductivity at the bottom and top of the electrode. Such cells tend not to have very high cycle life, and lose capacity as a result of poor charging and discharging.
While the addition of organic plasticizers may offer a solution to low ionic conductivity in polymer electrolytes, they necessarily introduce additional deleterious effects on other electrolyte properties such as stability in contact with the polymer matrix. Indeed, it is now known through manufacturers and suppliers of PVDF resins to the battery industry that PVDF is unstable in the organic solvents presently used in lithium ion polymer electrolyte batteries, dissolves to an extent, and that the instability worsens at elevated temperatures. The result is a breakdown in the PVDF mechanical integrity and loss of separator property, with the possibility of electrical shorting. Another problem found with PVDF is that the polymer swells and loses dimensional stability when it contacts liquid organic solvent. Consequently, the battery would exhibit poor electrode/electrolyte interface during thermal cycling and poor mechanical property of the gel compared to that of the polymer. In addition, polymer electrolytes based on such designs cannot be manufactured in very thin film forms to reduce overall resistance and, hence, cell resistance, since the polymer has insufficient strength to hold the liquid organic solvents in its matrix. For such a system to be fully functional it must be based on a thick film concept, which increases overall cell resistance and reduces energy density because of a reduction in the active components in the cell.
Thus, all of the aforementioned prior art techniques which have been employed in an effort to improve ionic conductivity, mechanical strength, safety, chemical stability, and cost reduction by simplifying the synthesis of polymer electrolytes, exhibit one or more substantial problems.
Accordingly, it is a principal object of the present invention to provide a base polymer material for a polymer electrolyte that is insoluble in the organic solvents presently used in lithium ion batteries and is highly stable with temperature.
Another object of the invention is to provide a base polymer material for a polymer electrolyte that exhibits little or no swelling characteristics when in contact with liquid organic solvents, compared to PVDF.
Another object of the invention is to provide a base polymer material that is predominantly amorphous in nature.
Still another object is to provide a base polymer material for a polymer electrolyte that is mechanically stronger than PVDF when in contact with liquid organic solvents. .
Still another object is to provide a base polymer electrolyte with ionic conductivity.
Yet another object of the invention is to provide polymer electrolyte compositions which are more conductive at lower levels of organic solvents than prior art polymer electrolyte-solvent compositions.
A further object of the invention is to provide polymer electrolyte compositions in which the solvent is immobilized in the polymer, to allow lithium ion batteries constructed from such compositions to be used in any orientation.
Another object of the invention is to provide polymer electrolyte compositions that can be manufactured in very thin film form, provide low resistance and excellent flexibility.
Still another important object of the invention is to provide a lithium ion battery with polymer electrolyte compositions described in the preceding enumerated objects.
A further object is to provide such lithium ion polymer electrolyte batteries with ultra-thin current collectors such as very thin metallic elements or metallized polymer substrates for improved energy density, power density, higher capacity utilization, higher cycle life, greater charge-discharge efficiencies, lower polarization, greater safety, and greater reliability, and which be produced at high speed, lower cost, and with improved form factors.
Another object of the invention is to coat the thin substrate with very thin active anode and cathode material.
A related object is to laminate the anode and cathode elements on both sides of the metallized polymer substrate material so as to yield a highly flexible electrode.
The electrolyte of the present invention is preferably a cationic conductor, is very flexible and somewhat dry, is of low cost, and in some preferred embodiments of the invention is constructed in very thin film format. Polymer electrolytes of this design can be combined with various negative electrodes such as an alkali metal, alkaline earth metal, transition metal, ion-insertion polymers, ion-insertion inorganic electrodes, carbon insertion electrodes, and tin oxide electrodes, among others, and with various positive electrodes such as ion-insertion polymers, and ion-insertion inorganic electrodes, to provide batteries and supercapacitors having high specific energy (Wh/kg) (gravimetric) and energy density (Wh/l) (volumetric), high cycle life, low self-discharge, and which provide improved safety.
One embodiment of a solid base polymer material of a polymer electrolyte of a lithium ion battery according to the invention is a thin film that includes a hybrid copolymer solid-solution homogeneous blend of at least two polymers, one selected from a polar group having pronounced solvent retention properties, and the other selected from a second group consisting of polyester (PET), polypropylene (PP), polyethylene napthalate (PEN), polycarbonate (PC), polyphenylene sulfide (PPS), and polytetrafluoroethylene (PTFE), or a combination of two or more thereof The specific polymer of the latter group and its concentration in the blend are selected to tailor at least one desired property of the base polymer material. In a preferred embodiment, the polymer selected from the polar group is PVDF. In a two polymer blend, the concentration of one is in a range from 1% to 99% by weight, and the remainder being the other. The base polymer material may include other substances such as an acrylate, polyethylene oxide (PEO), polypropylene oxide (PPO), poly(bis(methoxy-ethoxy-ethoxide))-phosphazene (MEEP), polyacrylonitrile (PAN), polymethylmethacrylate (PMMA), polymethyl-acrylonitrile (PMAN), etc.
In a method of preparing such a base polymer material, resins of the two or more polymer constituents, the one from the polar group being selected at least in part for its pronounced solvent retention properties, are blended preferably in a co-extrusion twin-screw process, to produce a hybrid melt-cast film. This film is then subjected to stretching by biaxial orientation in machine direction and transverse direction, to a desired final thickness, preferably in a range from 0.5 to 25 microns.
In another embodiment, an electrolyte-retaining base polymer material for a lithium ion battery is a solid-solution polymer thin film cast from a solution of PP, PVDF and cured acrylate monomer/oligomer from which a solvent in which those constituents were dissolved has substantially evaporated. A liquid or semi-liquid electrolyte solution containing a lithium salt is absorbed within the thin film. In a process of manufacture of this embodiment, the PP, PVDF and acrylate monomer/oligomer are dissolved in a hydrocarbon solvent to form a polymer solution, which is then cast in a thin film, in part by evaporation of the solvent. The film is then soaked in liquid electrolyte solution containing lithium salt, for absorption of the electrolyte within the film, and the acrylate monomer/ oligomer is cured by subjection to electron beam or ultraviolet radiation.
A dimensionally stable embodiment of hybrid base copolymer solid-solution blend film for a lithium ion battery, the film being capable of electrolyte retention without appreciable swelling, is produced by a method in which PVDF and PP are mixed homogeneously to form a copolymer blend thereof A very high surface area inorganic fillerxe2x80x94either fumed silica or aluminaxe2x80x94is then dispersed with a concentration in a range from about 0.1% to about 30% by weight into the copolymer blend to enhance the porosity and mechanical stability of the thin film into which the copolymer blend with inorganic filler is cast. Finally, after extrusion of the resin blend and biaxial orientation, the resultant film is soaked in a liquid solvent electrolyte for absorption and retention in the film. Preferably, the film is soaked in a mixture of ethylenecarbonate-diethyl carbonate (EC-DEC), EC-dimethyl carbonate (EC-DMC), PC-EC-DMC or PC-DEC, each of the solvent mixtures containing a lithium salt such as (by way of example) lithium hexafluorophosphase LiPF6, lithium perchlorate LiClO4, lithium tetrafluoro-borate LiBF4, lithium hexafluoroarsenate LiAsF6, lithium tetrachloroaluminate LiAlCl4, lithium trifluoromethane sulfonate LiCF3SO3, lithium bis(trifluoromethane sulfonyl) imide (lithium imide) LiN(CF3SO2)2, or lithium methide LiC(SO2CF3)3 for ionic conduction. Dispersion of the inorganic filler into the copolymer blend is performed during co-extrusion of the PVDF and PP.
Still another embodiment of the invention resides in a dimensionally stable, highly resilient, hybrid base copolymer blend electrolyte film of predominantly amorphous structure having mechanical strength when in contact with liquid solvent electrolyte, for a lithium ion battery. This embodiment preferably comprises a copolymer blend of PVDF and either PP or PEN, with the very high surface area inorganic filler dispersed therein. To generate high ionic conductivity of the hybrid copolymer electrolyte film at reduced levels of liquid organic solvent, a liquid polymer with liquid organic solvent electrolyte and lithium salt is introduced into the film, and the electrolyte is immobilized to allow molecules of the liquid polymer to trap molecules of the electrolyte into pores of the film. Preferably, the liquid polymer is cross-linkable, such as a polymer based on acrylates and PEO-based materials, and radiation curing is performed to cross-fink the liquid polymer for trapping of molecules. Alternatively, some immobilization of the liquid organic solvent electrolyte may be achieved by using a non-ionizable liquid polymer.
Any of these polymer electrolyte films may be used to form an electrochemical cell, particularly a lithium ion battery, by tightly sandwiching the film between thin, flexible active anode and active cathode layers.
For example, one embodiment of a thin film lithium ion battery is formed from a resilient flexible hybrid polymeric electrolyte thin film that includes a homogeneous blend of at least two polymers with inorganic filler dispersed therein to increase surface area and porosity of the hybrid film, impregnated with a semi-liquid or even dry solution of liquid polymer, organic solvent electrolyte and lithium salt; and a pair of spaced-apart flexible thin film electrodes, each including a polymer substrate having an adherent electrically conductive layer thereon, the hybrid film being tightly sandwiched between the pair of thin film electrodes. The polymer substrate of each of the anode and cathode is preferably selected from a group of polymers including PET, PP, PPS, PEN, PVDF and PE, and each polymer substrate is metallized to form the conductive layer thereon.
According to another aspect of the invention, a lithium ion battery electrode comprises an ultra thin film metal substrate for at least one of a cathode substrate and an anode substrate of a lithium ion battery, the ultra thin film metal substrate having a thickness in a range from about one micron to about 10 microns. The ultra thin film metallized polymer substrate includes a polymer material selected from a group of polymers including PET, PP, PPS, PEN, PVDF, and PE, and has a thickness in a range from about 0.5 micron to about 50 microns, thereby rendering it very flexible for ease of coating and handling, to avoid kinking and deformation thereon during manufacture of lithium ion batteries.
The lithium ion battery polymer substrate may comprise a layer of polymer material, and a low resistance metallization layer having a conductivity in a range from about 0.01 ohm per square to about 1 ohm per square overlying and adhered to a side of the polymer material. Preferably, the layer of polymer material has a non-metallized margin with a width in the range from about one mm to about three mm. Preferably, also, a low resistance metallization layer having a conductivity in the aforementioned range overlies and is adhered to each side of the polymer material, and both sides of the layer of polymer material have such a non-metallized margin present at the same edge of the layer of polymer material.
According to yet another aspect of the invention, a method of fabricating a thin film lithium ion rechargeable battery includes incorporating an ultra thin film metallized polymer substrate in the battery during fabrication thereof, wherein the polymer layer in the substrate has a thickness in a range from about 0.5 micron to about 50 microns, in conjunction with very thin film battery electrode/electrolyte structures having thickness less than 5 microns/10 microns, respectively, wherein the thickness of the metallization layer on the polymer layer is selected according to desired conductivity thereof, e.g., less than about 0.01 micron.
The invention also provides novel methods of coating an ultra thin film metallized polymer substrate for a thin film lithium ion battery with very thin film active anode material and active cathode material. One method comprises steps of milling each of the anode material and the cathode material in a separate solvent to reduce the particle size of the respective material, injecting respective ones of the materials directly onto the substrate at opposite sides thereof, and subsequently drawing each of the materials at opposite sides of the substrate into a thin film of desired thickness using wire wound rods or Mayer rods of different wire diameters to control wet slurry thickness.
Another coating method includes incorporating each of the materials into its own aerosol mix, spraying atomized aerosol of each material directly on respective opposite sides of the film substrate while moving said substrate past the points of aerosol spray at high speed, and curing the sprayed material either by drying or radiation. Yet another coating method comprises evaporating the respective electrode material directly onto respective opposite sides of the substrate.
Also according to the invention, a method of fabricating a thin film lithium ion battery involves laminating anode and cathode elements on respective opposite sides of a double-metallized polymer substrate, whereby to yield a highly flexible electrode structure for the battery. Non-metallized margins are provided on each of the anode and cathode elements on the opposite sides of the metallized polymer substrate, and metal is sprayed on opposite ends of the laminated metallized polymer substrate for terminations thereto. These techniques enable the provision of a ratio of substrate thickness to active electrode thickness less than about 0.5.